1. Field of the Invention
The present invention relates to an apparatus and a method for controlling communication and, in particular, to a communication control apparatus and a communication control method for performing packet communication.
2. Description of the Related Art
In recent years, with the advancement of communication systems, moving image data that require a relatively large bandwidth of data communication have been delivered to viewers via a communication line, such as the Internet. Thus, viewers can receive moving image data and view moving image.
In general, to deliver such moving image data, in particular, moving image data that is required to be displayed in real time, such as a live image, over a long period of time, a protocol called a transport protocol for real-time applications (RTP) is used. The RTP is designed for transferring speech data or moving image data in real time. The RTP is a protocol defined by the Internet Engineering Task Force (IETF) as RFC 1889 and RFC 1890.
As used herein, data transferred using an RTP protocol (e.g., speech data or moving image data) is referred to as a “stream” or “stream data” as needed.
In general, in real-time communication, such as RTP communication, although the reliability of communication data is not so high, a lower-layer protocol, such as a UDP/IP protocol, is used, since a high communication speed can be relatively easily obtained. RTP communication is designed to be performed using the UDP/IP protocol.
As described above, although communication using a UDP/IP protocol is suitable for a real-time application, an error, such as packet loss, easily occurs. If such an error occurs, part of moving image data may be lost. Accordingly, a variety of techniques for compensating for an error are proposed.
For example, a scheme for limiting the range affected by an error is embedded in the encoded moving image data in the encoding phase. Alternatively, if an error occurs, processing that makes loss of part of moving image data unnoticeable is performed when a moving image is displayed.
In addition, a more reliable communication line has been proposed. Thus, under particular conditions, the error rate is significantly decreased.
On the other hand, in a situation in which an error frequently occurs, it is important to reduce the occurrence of an error.
For example, Japanese Patent Laid-Open No. 2002-77260 describes a technology that reduces the occurrence of an error, such as packet loss. That is, Japanese Patent Laid-Open No. 2002-77260 describes a technology in which by delaying transmission timing points, transmission of packets at very short intervals is prevented. In this way, the occurrence of packet loss caused by data overflow in a buffer in a network path (e.g., a buffer in a router) can be reduced.
However, when packets are transmitted at predetermined time intervals (at predetermined transmission intervals), transmission may not be performed within a frame rate range of moving image data of a stream depending on a transmission rate based on a fixed time or the number of packets transmitted in a given period of time. In the existing technology, such a situation is not taken into account, and therefore, a transmission error may occur.